Depending on the circumstances it can be desirable and/or particularly important to be able to sense the presence of various gases which might be dangerous or explosive. These include carbon monoxide, carbon dioxide, propane, methane, as well as other potentially explosive gases.
A variety of sensors are known which can detect various gases. These sensors are based on different technologies and have different performance characteristics and different cost characteristics. One technology of ongoing interest is represented by electrochemical sensors. This class of sensors is potentially reliable and inexpensive.
Electrochemical sensors can be designed so as to be responsive to a gas of interest and to be highly sensitive. They respond to a gas of interest with a respective output current. However, such sensors have a zero output current failure mode and zero output current in the absence of the selected gas. Because there is no specific failure indicator, external circuits have to be designed to supervise these types of sensors.
It has been known to use electrical stimulus to apply a current to such sensors, to measure the sensor's signal over time, and calculate a capacitance value. This capacitance value can indicate that the sensor(s) has (have) degraded beyond a predetermined threshold, or, it can be an indication the sensor has been removed from the circuit. However, by itself, it does not indicate the sensitivity of the respective electrochemical sensor.
Another prior art method measures an electrical noise in a sensor output signal. A trouble condition or indication can be output if the noise level falls below a predetermined fixed threshold. This method is based in a known characteristic; that as gas concentration increases, the sensor(s) not only output a signal indicative thereof, they also exhibit increased noise. FIG. 1A is a graph of output noise vs. gas concentration in parts per million which illustrates this characteristic. FIG. 1B illustrates exemplary response of an electrochemical sensor to a pulse of 100 ppm of CO. FIG. 1C illustrates increasing noise in response to exposure to the CO. However, this method does not teach maintaining the sensitivity. It only provides an indication of a failed sensor relative to a fixed threshold.